1. Field of the Invention
The present invention relates to a collimator lens usable in a light-scanning apparatus such as copier, laser printer, or the like which scans a laser beam in order to record or display an image; and, in particular, to a collimator lens for converting a divergent luminous flux emitted from a light source such as semiconductor laser into a parallel luminous flux, and a light-scanning apparatus using the same.
2. Description of the Prior Art
Various kinds of light-scanning apparatus such as copier, laser printer, and the like for scanning a laser beam in order to record or display an image have conventionally been known.
Such a light-scanning apparatus is configured such that a laser beam emitted from a semiconductor laser is converted by a collimator lens into a parallel luminous flux, which is then deflected in response to the rotation of a rotary polygon mirror, and thus deflected luminous flux is focused by an f.theta. lens onto an imaging surface.
Here, as the collimator lens, those of a two-group/two-element configuration have been known as disclosed, for example, in Japanese Unexamined Patent Publication Nos. 58-14109, 58-38915, 61-279820, 61-273520, and 2-73324, so as to reduce the weight and dimensions of lens.
In general, it is necessary for the collimator used in such a light-scanning apparatus to be a bright lens system in order to enhance the efficiency of utilization of the light from the light source and increase the illuminance on the photosensitive drum surface. As a consequence, the collimator lens would have a numerical aperture greater than that of f.theta. lens systems in general, whereby aberrations generated thereby are likely to increase. Therefore, in the collimator lens, it is necessary to favorably correct these aberrations such as wavefront aberration.
Meanwhile, as the light-scanning apparatus, there has been known a multibeam system which uses a plurality of light sources so as to carry out multibeam scanning, thereby achieving a higher scanning speed and enabling a plurality of different information items to be recorded at the same time in a single scanning operation. In the case where this multibeam system is employed or the like, it is desirable that aberrations be favorably corrected within the range of about 2 degrees in terms of half angle of view .omega..
In the collimator lenses disclosed in the above-mentioned publications, most of examples have a narrow angle of view, for instance, such that their performances with respect to off-axis light are taken into consideration only for an angle of view on the order of installation errors. Aberrations are so large in examples with a large angle of view that it is difficult for the collimator lenses to be employed in a multibeam scanning optical system having a plurality of light sources disposed on a plane perpendicular to the optical axis, for example.
Further, in the collimator lenses disclosed in the above-mentioned publications, back focus, i.e., the distance from the lens on the light source side to the light source, is short, i.e., about 0.4f to 0.6f.
If the back focus is short, however, then the collimator lens is disposed near the light source (semiconductor laser or the like), whereby the temperature of the collimator lens is likely to increase due to the heat from the light source. Therefore, it is desired that the distance from the collimator lens to the light source be made greater in order for the collimator lens to be less susceptible to the heat from the light source.